Semiconductor transistors have been fabricated by a process known as collector diffused isolation (CDI). In one form of such device an N+ buried layer is first diffused into a P substrate, the substrate being approximately 1 to 10 ohm-centimeters and the sheet resistance of the N+ buried layer being of a value approximately equal to 20 ohms per square. A thin P epitaxial film layer is grown over the P substrate and N+ buried layer to a thickness of about 2 to 5 microns with a concentration of about 0.5-5.0 ohm-cm. Thereafter a ring-like or frame-like N+ isolation region is diffused into the thin P epitaxial film layer with a deep diffusion down through the P epitaxial layer into the N+ buried layer, leaving a central P epitaxial film region surrounded by the N+ isolation ring.
A P base is then diffused into the P epitaxial film layer within this N+ isolation ring and thereafter an N+ emitter region is diffused into the P base region. At the time of the diffusion of the N+ emitter region, an N+ collector contact region is diffused into the N+ isolation ring. The N+ isolation ring has a sheet resistance of about 2-10 ohms per square, the sheet resistance of the base region is about 100-300 ohms per square, and the sheet resistance of the emitter area is about 2-10 ohms per square.
This process differs from a typical process in that a thin P epitaxial film layer is grown on the P substrate and N+ buried layer rather than the typical N epitaxial film region, and the P base and N plus emitter are formed in this P epitaxial region rather than in an N epitaxial film region. The collector for this device comprises the N+ buried layer and the N+ isolation area to which a surface contact is made by the N+ collector contact diffusion. This distinquishes from the normal situation in which the collector region is formed by the N epitaxial film region grown over the substrate.
In a second version of the collector diffused isolation device, the above described process is followed up to the point where the P base is to be diffused and, in this second process, no such P base diffusion takes place; rather an N+ emitter diffusion is made directly into the P epitaxial film layer. The N collector region is formed by the N+ buried layer and N+ isolation ring surrounding the device.
Two principal disadvantages exist with such collector diffused isolation process devices, one disadvantage being a low breakdown voltage between collector and base of about ten volts. A more serious disadvantage for such devices is that no lateral PNP devices can be fabricated by this NPN structure since the P epitaxial film region would form a short between the P collector and P emitter, and, in effect, form a PPP device, which is, of course, unsuitable.